In a digitally controlled printing system, such as an inkjet printing system, a print medium is directed through a series of components. The print medium can be a cut sheet or a continuous web. A web or cut sheet transport system physically moves the print medium through the printing system. As the print medium moves through the printing system, liquid, for example, ink, is applied to the print medium by one or more printheads through a process commonly referred to a jetting of the liquid. The jetting of liquid onto the print medium introduces significant moisture content to the print medium, particularly when the system is used to print multiple colors on a print medium. Due to its moisture content, the print medium expands and contracts in a non-isotropic manner often with significant hysteresis. The continual change of dimensional characteristics of the print medium often adversely affects image quality. Although drying is used to remove moisture from the print medium, drying too frequently, for example, after printing each color, also causes changes in the dimensional characteristics of the print medium that often adversely affects image quality.
FIG. 1 illustrates a portion of the print medium 112 as the print medium passes over two rollers 108 that support the print medium in accordance with the prior art. During an inkjet printing process, the print medium can expand as the print medium absorbs water-based inks applied to it. When the direction of expansion is in a direction that is perpendicular to the direction of medium travel 100, it is often referred to as expansion in the crosstrack direction 102. Typically, the wrap of the print medium 112 around a roller 108 of an inkjet printing system produces sufficient friction between the print medium and the roller that the print medium is not free to slide in the crosstrack direction even though the print medium is expanding in that direction. This can result in localized buckling of the print medium away from the roller to create lengthwise ripples, also called flutes or wrinkles, in the print medium. Ridges or flutes 104 can be produced in the print medium 112 due to expansion of the print medium in the crosstrack direction 102 because the print medium cannot slip on the rollers 108. This wrinkling of the print medium during the printing process often leads to permanent creases forming in the print medium that ultimately affect image quality and are considered a print defect.
Multiple printheads are typically located and aligned by a support structure to form a linehead, with the linehead located over the print medium. In many such systems, the support structure of the linehead disposes the printheads in two or more rows; the rows disposed parallel to each other and aligned in the crosstrack direction. To prevent the print medium from fluttering, or vibrating up and down in the print zone, the print medium is supported by a roller that is aligned with the print line of each row of printheads. It is not uncommon for the bottom face of the support structure to become wet, either due to condensation from the moist air produced by the printing process or due to mist drops created by the print drops striking the print medium.
It has been found that, under some printing conditions, the flutes in the print medium are sufficiently tall that the top of the flutes can contact the bottom face of the support structure. When this occurs, the moist ink on the flutes can be smeared by the contact. Additionally, the moisture on the bottom of the support structure can be transferred to the print medium. The result is a degradation of the print quality. There remains a need in the art for a printing system that reduces the flutes or wrinkles in the print medium and prevents smearing of the ink from the medium coming into contact with the support structure of the lineheads.